In some gas turbine engines a turbine rotor comprises a plurality of axially spaced turbine discs. The turbine discs are fastened together at a first radially inner position by a plurality of axially extending bolts, which extend through axially extending projections on the turbine discs, and associated nuts. The turbine discs have axially extending sealing members to define seals at a second radially outer position.
A problem with this turbine rotor is that if the turbine discs are not clamped, or fastened, together sufficiently tightly there is a significant axial gap between the axially extending sealing members and this allows the turbine rotor and hence the gas turbine engine to vibrate. Additionally, if the sealing members are machined out of tolerance, or if the sealing members are worn, there may be a significant axial gap between the axially extending sealing members and this allows the turbine rotor and hence the gas turbine engine to vibrate.
Thus there is a need to ensure that the there is no gap, or substantially no gap, between the sealing members so as to reduce the vibration of the turbine rotor.
It is known to detect defects, cracks or flaws, in articles by injecting a coherent high frequency, over 1 MHz, ultrasonic signal into the article and detecting the reflection of the coherent high frequency ultrasonic signal from the defect. By measuring the time for the high frequency ultrasonic signal to travel from the point of injection to the defect and back the location of the defect is determined.